Push-to-Talk-over-Cellular allows a user or telecommunication terminal to transmit voice to one or more receivers simultaneously using a half-duplex method when the receiver or a plurality of receivers has/have been selected and when a Push-to-Talk-over-Cellular key (PoC key) provided specifically for this purpose on the telecommunication terminal has been pressed.
A half-duplex method is understood to mean a transmission method in which only one unidirectional useful data transmission can only take place at a time, i.e. for example only the sender can speak and the receivers cannot interrupt him. In this case, the voice data, also called “talk bursts”, are actually distributed over the telecommunication network while the voice data are being spoken and while the voice data are being transmitted from the sender telecommunication terminal. Thus, from the point of view of the user, Push-to-Talk-over-Cellular is similar to conventional CB radio, but with the extension that the sender is able to speak to receivers throughout the world that are able to be reached using the communications engineering of at least one mobile radio communication network. A prerequisite in this case is that the receiver or the receivers, i.e. the respective receiver telecommunication terminals, are also registered in the mobile radio communication network at this time, in other words are “online”.
Conventionally, the flow of signalling takes the following appearance:
Subscriber A, i.e. the subscriber on the sender telecommunication terminal, presses the button provided for activating PoC on the sender telecommunication terminal.                Following an appropriate query message which the sender telecommunication terminal uses to ask the PoC server computer for a talk right and which message is now transmitted to the PoC server computer, the sender telecommunication terminal might receive notification from the PoC server computer that subscriber B cannot be reached, and is asked whether it wishes to leave a message.        Subscriber A can ignore this message or can again press a button (key) provided for the functionality below on the telecommunication terminal, for example the PoC button (PoC key), and hence activate that his message is recorded in the PoC box of subscriber B.        After a time, subscriber B can be reached again, which the PoC communication service and particularly the PoC server computer can ascertain by monitoring the “Presence Info” parameter of subscriber B, for example.        The PoC communication service now initiates a 1-to-1 PoC session with subscriber B and notifies subscriber B that a stored message from subscriber A is stored in the PoC box. The stored message is then transmitted to subscriber B, i.e. to the receiver telecommunication terminal.        Subscriber B can then, having received the message from the PoC server computer, press his PoC button (PoC key) on the receiver telecommunication terminal so as implicitly to set up a 1-to-1 PoC communication session with subscriber A.        